Photovoltaic devices provide a nonpolluting, silent and reliable source of electrical power. A number of techniques have now been developed for the fabrication of thin film photovoltaic devices which can be manufactured at a relatively low cost and over large areas.
Thin film photovoltaic devices, particularly those manufactured from semiconductor alloys including group IV elements, are frequently fabricated in a N-I-P configuration. In devices of this type, a body of substantially intrinsic semiconductor material is interposed between oppositely doped layers of semiconductor material. Absorption of a photon by the semiconductor material in the intrinsic layer causes the generation of an electron/hole pair, and these carriers are swept out of the intrinsic layer by a built-in field established by the presence of the P doped and N doped layer, so as to be collected by electrodes associated with the device. It is to be understood that within the context of this disclosure, the intrinsic layer of the N-I-P type device may be very slightly P type or N type; however, within the device such layers will still function as intrinsic layers and hence are referred to as substantially intrinsic. It is also to be understood that such cells are often fabricated in an inverted configuration, and remarks made regarding N-I-P cells are also applicable to P-I-N cells as well.
Since, in an N-I-P type photovoltaic device photo generation of carriers takes place within the intrinsic body, it is desirable that passage of light thereinto be as unimpeded as possible. Also, it is desirable that the electrical conductivity of the various layers of the device be as high as is practically possible, so as to minimize resistive losses. Accordingly, it is preferred that, at least the light incident layer of an N-I-P type photovoltaic device be fabricated from a high transparency, high conductivity semiconductor material. U.S. Pat. No. 4,600,801 discloses a highly conductive, highly transparent P doped, microcrystalline semiconductor alloy material having particular utility in N-I-P type photovoltaic devices, and the disclosure thereof is incorporated herein by reference. As specifically disclosed therein microcrystalline materials are a type of disordered material which are distinguishable from amorphous materials insofar as they exhibit a threshold volume fraction of crystalline inclusions at which substantial changes in key parameters, including electrical conductivity, band gap and absorption constant occur.
The concept of microcrystalline materials exhibiting a threshold volume fraction of crystalline inclusions at which substantial changes in key parameters occur can best be understood with reference to the percolation model of disordered materials. Percolation theory, as applied to microcrystalline disordered materials, analogizes properties such as the electrical conductivity manifested by microcrystalline materials, to the percolation of a fluid through a nonhomogeneous, semipermeable medium such as a gravel bed. Microcrystalline materials are formed of a random network which includes low conductivity, highly disordered regions of materials surrounding randomized, highly ordered crystalline inclusions having high electrical conductivity. Once these crystalline inclusions attain a critical volume fraction of the network (which critical volume will depend, inter alia, upon the size and/or shape and/or orientation of the inclusions), it becomes a statistical probability that said inclusions are sufficiently interconnected so as to provide a low resistance current path through the network. Therefore, at this critical or threshold volume fraction, the material exhibits a sudden increase in conductivity. This analysis (as described in general terms relative to electrical conductivity herein) is well known to those skilled in solid state theory and may be similarly applied to describe additional physical properties of microcrystalline materials such as optical gap, absorption constant, etc.
The onset of this critical threshold value for the substantial change in physical properties of microcrystalline materials will depend upon the size, shape and orientation of the particular crystalline inclusions, but is relatively constant for different types of materials. The shape of the crystalline inclusions is critical to the volume fraction necessary to reach the threshold value. There exist one-dimensional, two-dimensional and three-dimensional models which predict the volume fraction of inclusions necessary to reach the threshold value, these models being dependent on the shape of the crystalline inclusions. For instance, in a one-dimensional model (which may be analogized to the flow of charge carriers through a thin wire), the volume fraction of inclusions in the amorphous network must be 100% to reach the threshold value. In the two-dimensional model (which may be viewed as substantially conically shaped inclusions extending through the thickness of the amorphous network), the volume fraction of inclusions in the amorphous network must be about 45% to reach the threshold value. Finally in the three-dimensional model (which may be viewed as substantially spherical shaped inclusions in a sea of amorphous material), the volume fraction of inclusions need only be about 16-19% to reach the threshold value. Therefore, amorphous materials may incorporate crystalline inclusions without being microcrystalline as the term is defined herein.
In many instances it has been found advantageous to fabricate photovoltaic devices comprised of a plurality of individual cells stacked in an optical and electrical series relationship so as to produce tandem photovoltaic devices. In some instances the band gaps of the materials comprising the individual cells of the tandem device are varied so as to produce a spectrum splitting device in which the uppermost cells thereof are responsive to short wavelength illumination and relatively transparent to longer wavelength illumination, while the bottom cells of the stack are tailored to absorb and respond to longer wavelength illumination. The high transparency and good electrical conductivity of microcrystalline materials makes them useful in the fabrication of tandem N-I-P type photovoltaic devices, and tandem devices incorporating microcrystalline P layers are shown in U.S. Pat. No. 4,609,771, the disclosure of which is incorporated herein by reference.
Tandem N-I-P type devices include an internal junction between the N layer of a first cell and the P layer of a succeeding cell. This junction is not a photovoltaicly productive junction, and the inventors hereof have reasoned that device performance could be improved if the electrical conductivity and/or transparency of the N layer could be improved. A decrease in optical absorption of the N layer will permit more light to pass through to the photo generative portion of the cell, thereby improving short circuit current. The increase in conductivity will improve the fill factor of the cell by optimizing the tunnel junction between the N and P layer, and will also improve the open circuit voltage of the cell. Overall, it is expected that inclusion of a microcrystalline N layer into an N-I-P type cell, particularly one incorporated into a tandem device, will improve the overall performance of the device.
In order to verify this hypothesis, the inventors hereof prepared a series of N-I-P type photovoltaic devices, each comprised of a layer of N doped silicon alloy material supported upon a substrate electrode, a body of intrinsic silicon alloy material deposited thereatop, and a layer of microcrystalline P type silicon alloy material disposed upon the intrinsic layer. In order to approximate the effects in a tandem device, the inventors hereof deposited an additional layer of N doped material atop the light incident, P layer of the aforementioned devices. In a first experiment, the additional N layer was a layer of substantially amorphous silicon alloy material. In a second experiment, the second N layer was a microcrystalline layer fabricated under deposition conditions favoring the preparation of a slightly microcrystalline layer; and a third experiment involved the deposition of a highly microcrystalline layer of N doped material atop the P layer of the aforementioned cell. The operational parameters of each of the foregoing devices were then measured and are summarized hereinbelow in Table 1 where J.sub.sc is the short circuit current, Q.sub.550 is the quantum efficiency at 550 nm, V.sub.oc is the open circuit voltage, and FF is the fill factor.
TABLE 1 ______________________________________ Sample J.sub.sc Q.sub.550 V.sub.oc FF ______________________________________ 1 11.2 0.70 0.916 0.592 2 11.5 0.72 0.948 0.643 3 11.9 0.74 0.938 0.610 ______________________________________
It will be noted from the foregoing that cell performance does in fact increase as the top N layer becomes microcrystalline.
Having conducted this experiment, the inventors hereof then proceeded to fabricate a series of tandem devices from stacked N-I-P type cells. In a first device, indicated by sample no. 4 in Table 2 hereinbelow, the N layer which was in contact with the P layer was fabricated from amorphous, N doped silicon alloy material, corresponding generally to the amorphous material of sample 1 of Table 1. In a second device of this experimental series, indicated by sample no. 5 in Table 2, the corresponding N layer was fabricated from a microcrystalline material having a relatively high degree of microcrystallinity and corresponding generally to that of sample 2 of Table 1. A third device of this experimental series is indicated by sample no. 6 in Table 2 hereinbelow and includes an N layer having a high degree of microcrystallinity and corresponding generally to that of sample 3 of Table 1. The thus fabricated devices were tested and the open circuit voltage and fill factors thereof are summarized in Table 2 hereinbelow.
TABLE 2 ______________________________________ Sample V.sub.oc FF ______________________________________ 4 1.804 0.652 5 1.755 0.659 6 1.657 0.666 ______________________________________
As will be seen from Table 2, the results of this experimental series were surprising insofar as the open circuit voltage of the tandem photovoltaic devices actually dropped as the N layer became more microcrystalline. This result appears counterintuitive, and at odds with the data from the first experimental series as summarized in Table 1. While the inventors hereof do not wish to be bound by speculation, it is postulated that the loss of open circuit voltage is resultant from a mismatch in the band gaps of the microcrystalline N and the amorphous I layer; which, because of the relative positions of the fermi levels therein produces an overall lowered cell voltage as compared to the voltage produced when an amorphous N layer is joined to an amorphous I layer.
In any instance, it is clear from the foregoing that inclusion of a highly conductive, highly transparent, microcrystalline N layer in an N-I-P type photovoltaic device is expected to provide enhanced device output, as compared to when an amorphous layer is included. However, while such benefits have heretofore been demonstrated in single cell devices, tandem devices fabricated with microcrystalline N layers actually show a reduced output, primarily as a result of lowered device voltage. The present invention provides a particular configuration of photovoltaic device which attains the benefits resultant from the high transparency and high conductivity of a microcrystalline body of N doped material while preventing the loss of voltage as demonstrated in the second experimental series detailed hereinabove. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.